RELATIVE PREDATION POTENTIALS OF SCYPHOMEDUSAE, CTENOPHORES AND PLANKTIVOROUS FISH ON ICHTHYOPLANKTON IN CHESAPEAKE BAY

The relative predation potentials on ichthyoplankton of the scyphomedusa Chrysaora quinquecirrha, the ctenophore Mnemiopsis leidyi and the bay anchovy Anchoa mitchilli from Chesapeake Bay, USA, were estimated in 3.2 m3 in situ mesocosm enclosures and in 1.0 m3 laboratory tanks. For all 3 predators, averaged predation mortality (d-1) and volume-specific clearance rates (l d-1 ml-1) were higher and less variable when bay anchovy eggs were prey than when goby Gobiosoma bosci larvae [3.0 to ca 10.0 mm standard length (SL)] were prey. The smallest larvae (3.0 to <5.5 mm SL) were more vulnerable than eggs or larger larvae. Averaged mortality rates per scyphomedusa (0.78 and 0.32 d-1 on eggs and larvae respectively) were 7 times higher than those per ctenophore (0.11 and 0.04 d-1), and almost 2 times higher than those per bay anchovy (0.37 and 0.21 d-1). However, volume-specific clearance rates by the relatively small bay anchovy predators were higher (ca 500 l d-1 ml-1) than those of the gelatinous predators. The volume-specific clearance rates of the ctenophore and medusa were only 4 and 7 % respectively of that for the anchovy. Combined species results suggest that these predators may consume 20 to 40 % daily of the fish eggs and larvae in mid-Chesapeake Bay. The scyphomedusa potentially is the most important predator on summer ichthyoplankton due to its overall abundance, high clearance rates and temporal co-occurrence with vulnerable life stages of fish.